1. Field of the Invention
The present invention provides a coupling assembly for plastic pipe, and more particularly a coupling assembly in which the coupled pipe has enhanced axial tension strength resulting in a pipe and coupling connection that can withstand high axial loads in tension, and that require no additional means for maintaining the coupling assembly in a coupled state. This coupling assembly is particularly useful in applications that require that multiple coupled lengths of pipe be pulled long distances through underground boreholes while maintaining a seal between the coupled lengths of pipe without disconnecting. The present invention provides an inexpensive plastic coupling assembly for conduit or pipe comprising a tubular component, a coupler, an annular locking strap, and an annular sealing member, the combination being easy to assemble and disassemble and allowing at least one thousand feet of such pipe to be pulled through an underground borehole without the use of additional fasteners to maintain the coupled state.
2. Description of Related Art
Fiber optic transmission lines and other cables have increasingly replaced metallic electricity-conducting wires. For various reasons, it has been desirable to bury fiber optic cables, utility cables and utility pipes underground. To protect the cables and pipes while underground, the cables or pipes may be placed in a conduit or duct. See, e.g., U.S. Pat. No. 5,027,864 to Conti, et al. The cables may be placed in an inner duct, that, in turn, is pulled through a larger outer duct. See also U.S. Pat. No. 5,087,153, to Washburn.
One preferred material for underground duct applications is PVC pipe or tubing, which is normally supplied in lengths of 4, 10 or 20 feet, but may be supplied in other lengths. Each such length of pipe must be coupled to adjacent lengths by means of a coupling assembly. A single 20 foot length of 4-inch PVC pipe weighs approximately 45 pounds. Thus, a tunnel one thousand feet long will require that some of the coupling assemblies bear an axial tensile force of at least several thousand pounds due to the combined effects of pipe weight, frictional drag resulting from the pipe walls contacting the walls of the borehole while the conduit is pulled, or contact with other obstructions. When this several thousand pounds is applied over the surface area contacted by a locking mechanism in a coupling assembly, the pressure borne by the locking portion of the coupling assembly may be close to the tensile strength of PVC. Previously known PVC coupling assemblies could not bear such loads, or required additional fasteners. Prior art augmentation of such coupling assemblies greatly increased the difficulty, expense and even danger of using coupled lengths of plastic pipe for such applications. Coupling assemblies made from other materials such as metal were unsatisfactory for reasons such as weight, limitations of the materials, such as the proneness to corrosion of some metals (e.g., aluminum or steel); or expense (e.g., stainless steel).
Continuous lengths of plastic tubing have been used for underground duct applications. The method for installing this type of conduit into a horizontal borehole is by pulling long lengths of the tubing from a spool through the borehole. The most often used plastic tubing is polyethylene ("PE"), supplied on large, heavy spools, each containing some 600 feet of tubing. Such tubing generally has a wall thickness of 0.320 inch in a four-inch diameter. While inexpensive and widely available, PE tubing suffers from several drawbacks. Adjacent lengths must be butt sealed together by appropriate welding with a specialized apparatus. Both the tensile strength and crush resistance of PE tubing are less than that of a material such as PVC pipe. Such tubing frequently suffers from increased ovality due to the flattening effect of being coiled on the spool. A contractor installing the tubing must feed each spool from a specially designed apparatus, upon which each spool must be mounted in turn. When the end of the spooled tubing is released, it can dangerously whip around, potentially causing serious injury to workers and others. In a related effect, PE has a considerable "memory" of the curvature it has been forced to adopt while on the spool, as a result of which the PE tubing retains a strong tendency to curl after it is removed from the spool. Finally, tubing is often wasted if the length of the borehole does not equal the length of a multiple number of spools of tubing.
PVC pipe lengths have several advantages over continuous lengths of coiled tubing. With PVC pipe, the exact number of lengths needed for a job can be stacked together and delivered in an ordinary flatbed truck minimizing time, manpower, equipment and wasted material. PVC pipe has greater tensile strength and crush resistance than polyethylene tubing, and has better resistance to developing ovality. Because the lengths have not been forced to bend prior to use, they do not suffer from "memory" problems found with PE tubing.
Use of PVC pipe has not been without disadvantages. Previously known PVC pipe couplings typically required augmentation. Thus, numerous steps were performed in prior art methods to assemble the coupled lengths of PVC pipe. One method requires cementing the joint together after it is assembled. The typical coupling for such PVC pipe is a bell and spigot type coupling, in which each length of pipe has one end slightly belled outward (the coupler) and the other end not belled (the tubular component). The belled end is enlarged to a degree sufficient to allow a non-belled end of an adjacent length to enter, forming a sealed coupling when properly cemented together.
The cementing process includes all the known difficulties associated with PVC cement, including the use of noxious, hazardous solvents and the time required for the PVC cement to cure. The most serious drawback of this type of glued-together coupling for use in applications requiring installation by pulling through a substantially horizontal borehole is the lack of resistance to axially applied tension when in the coupled state. This drawback has only previously been overcome by augmenting the PVC cement with other fastening means, typically radially inserted screws. In order to securely attach the lengths together, screws such as self-tapping metal screws are inserted radially into the coupling assembly adding an additional step, equipment, and personnel. But, even with this improvement, additional time for the glue to cure is needed in advance of the time when the pipe is to be pulled through the tunnel. As a result, it is normally necessary to pre-assemble several hundred feet or the entire string of glued- and screwed-together PVC pipe, before the pulling process can be started.
An additional drawback of using bell and spigot coupling assemblies results from the diameter of the bell end, which is larger than the remainder of the pipe. This increased diameter makes the installation of the pipe through the underground borehole more difficult due to increase drag especially when rocks, roots, or other obstructions are encountered. The screw heads also increase the installation difficulties because they extend radially outwardly from the bell further increasing the outside diameter of this portion of the pipe and provide a location for encountering snags with obstructions.
The present invention provides a simpler, stronger, and easier to assemble coupling assembly than any known heretofore.